The Venus Monitoring Camera on the Venus Express captured this false-color image of cloud features seen on Venus. The image was captured from a distance of 18,641 miles (30,000 kilometers) in 2011. // ESA/MPS/DLR/IDA

The most detailed record of cloud motion in the atmosphere of Venus chronicled by the European Space Agency’s (ESA) Venus Express has revealed that the planet’s winds have steadily been getting faster over the last six years.

Venus is well-known for its curious super-rotating atmosphere, which whips around the planet once every four Earth days. This is in stark contrast to the rotation of the planet itself — the length of the day — which takes a comparatively laborious 243 Earth days.

By tracking the movements of distinct cloud features in the cloud tops some 43.5 miles (70 kilometers) above the planet’s surface over a period of 10 venusian years (six Earth years), scientists have been able to monitor patterns in the long-term global wind speeds.

When the Venus Express spacecraft arrived at the planet in 2006, average cloud-top wind speeds between latitudes 50° on either side of the equator were clocked at roughly 186 mph (300 km/h). The results of two separate studies have revealed that these already remarkably rapid winds are becoming even faster, increasing to 249 mph (400 km/h) over the course of the mission.

“This is an enormous increase in the already high wind speeds known in the atmosphere. Such a large variation has never before been observed on Venus, and we do not yet understand why this occurred,” said Igor Khatuntsev from the Space Research Institute in Moscow.

Khatuntsev’s team determined the wind speeds by measuring how cloud features in images moved between frames. They painstakingly tracked more than 45,000 features by hand and another 350,000 features automatically by using a computer program.

In a complementary study, a Japanese-led team used their own automated cloud-tracking method to derive the cloud motions.

On top of this long-term increase in the average wind speed, however, both studies also have revealed regular variations linked to the local time of day and the altitude of the Sun above the horizon, and to the rotation period of Venus.

One regular oscillation occurs roughly every 4.8 days near the equator, and scientists think it’s connected to atmospheric waves at lower altitudes.

But the research also unveiled some harder-to-explain curiosities.

“Our analysis of cloud motions at low latitudes in the southern hemisphere showed that over the six years of study, the velocity of the winds changed by up to 70 km/h [about 40 mph] over a time scale of 255 Earth days — slightly longer than a year on Venus,” said Toru Kouyama of the Information Technology Research Institute in Ibaraki, Japan.

The two teams also saw dramatic variations in the average wind speeds between consecutive orbits of Venus Express around the planet.

In some cases, wind speeds at low latitudes varied such that clouds completed one journey around the planet in 3.9 days, while on other occasions they took 5.3 days.

Scientists currently have no explanation for either of these variations or for the long-term overall increase in wind speeds.

“Although there is clear evidence that the average global wind speeds have increased, further investigations are needed in order to explain what drives the atmospheric circulation patterns that are responsible, and to explain the changes seen in localized areas on shorter timescales,” said Håkan Svedhem, ESA’s Venus Express project scientist. “The atmospheric super-rotation of Venus is one of the great unexplained mysteries of the solar system. These results add more mystery to it, as Venus Express continues to surprise us with its ongoing observations of this dynamic, changing planet.”